Various forms of antennas have conventionally been used as an antenna for receiving various broadcast waves such as television broadcast or FM broadcast. For example, a dipole antenna or a Yagi-Uda antenna is often used for receiving television broadcast and FM broadcast.
On the other hand, the various broadcast waves have increasingly being received in a room, in a car or during travel on foot. The antenna used in such cases needs to be easily handled, for example, for assembly or installation.
Such an easily-assembled or easily-handled antenna is typified by a dipole antenna that is implemented by the antenna elements that are simply structured. A cobra antenna is known as an embodiment of the dipole antenna. The cobra antenna is used with some turns of a coaxial wire around a ferrite core (for example, Non-patent Document 1).
FIG. 5 is a view for showing an exemplary cobra antenna that has been produced by modifying a dipole antenna. As shown in FIG. 5, a cobra antenna 100 includes a central conductor (core wire) 300 and a ferrite core 400. On the assumption that the radio wave to be received has a wavelength of λ, the central conductor 300 is λ/4 in length and is connected, as an upper element, on a feeding point 200. The ferrite core 400 is provided under and λ/4 away from the feeding point 200. A coaxial cable (coaxial wire) 500 is wound around the ferrite core 400. Although the coaxial cable 500 is wound 3 times in FIG. 5, the number of turning (the number of winding) does not necessarily need to be three times. The number may be once or twice.
When the coaxial wire is wound around the ferrite core 400 three times or more, the impedance tends to drastically decrease regardless of the size of the ferrite over about the frequency of 100 MHz. For example, it has been reported that, when the number of winding is once, the impedance of the antenna tends to increase even though the frequency exceeds 100 MHz; however, when the number of winding is three times, the impedance drastically decreases.
In the cobra antenna shown in FIG. 5, a choke coil is formed by a ferrite core 300 and the coaxial cable 500 wound around the ferrite core. The choke coil separates a feeder part below the ferrite core 400 so that a λ/4 dipole antenna can easily be formed. An egg-shaped glass or the like is attached to the upper core wire 300 of the dipole antenna for insulation so that the antenna can be hung from a tree branch or a wooden frame. This can facilitate the installation of an antenna. A cobra antenna structured in such a manner can also be applied to an antenna of a car-mounted mobile device.